Antoni made his K-1 debut at the K-1 World Grand Prix 2001 Preliminary Scandinavia on June 9, 2001 where he was participating in an eight-man elimination tournament.
In the semi-finals he faced another Swede, Wisam Feyli, who he defeated by a third round unanimous decision, thus booking his place in the tournament final.
In the final match Antoni faced yet another Swede, Larry Lindwall, but was unable to win the tournament, losing instead by second-round technical knockout.
On September 29, 2002 Antoni would face fellow Dutch kickboxer and future K-1 Grand Prix Champion Remy Bonjasky at the It's Showtime – As Usual / Battle Time event in the Netherlands.
Hardonk also revealed that he will be partnering with his instructor since 2004, Henry Akins, to become a full-time trainer at his gym "Dynamix Martial Arts" located in Santa Monica.